Conventionally, thermoelectric generation of electricity is known in which the thermoelectric elements are disposed between a heat exchanger at a higher temperature part and another heat exchanger at a lower temperature part to generate electricity. The thermoelectric element is an application of a thermoelectric effect to be called Seebeck effect. In the case where a semiconductor material is used as a thermoelectric material, the thermoelectric power module is configured by electrically connecting a thermoelectric element formed of a P-type semiconductor thermoelectric material and another thermoelectric element formed of an N-type semiconductor thermoelectric material via an electrode.
Such a thermoelectric power module has a simple structure and can be easily treated, and stable characteristics can be retained. Therefore, research work thereof has been widely progressed toward application for the thermoelectric generation of electricity in which electricity is generated by utilizing heat in a gas discharged from an engine of a car, a furnace of a factory, and so on.
Generally, the thermoelectric power module is used in a temperature environment where a difference between a temperature (Th) at a higher temperature part and a temperature (Tc) at a lower temperature part becomes large in order to obtain high thermoelectric conversion efficiency. For example, a thermoelectric power module employing a typical bismuth-tellurium (Bi—Te) based thermoelectric material is used in a temperature environment where a temperature (Th) at the higher temperature part becomes 250° C. to 280° C. at maximum. Accordingly, in the case where nickel plating is applied to an electrode in order to improve solder wettability or the like of the electrode, diffusion of nickel into a solder layer or oxidation of nickel becomes a problem.
As a related art, Japanese patent application publication JP-P2004-14766A (paragraphs 0005-0007) describes a conventional thermoelectric module in which an electroless plating film of a Ni—P or Ni—B base alloy is formed between a thermoelectric element and an electrode in order to prevent diffusion of solder, and a problem that a resistivity of the electroless plating film is high, and when an electric current flows in each thermoelectric element, heat due to the resistance is generated in the plating film and heat is also generated in a heat absorption part, and as a result, performance of the thermoelectric module is lowered than a theoretical value depending on physical properties of a material of the thermoelectric element.
In order to solve this problem, JP-P2004-14766A discloses a thermoelectric module in which thermoelectric elements are connected in series or parallel via upper electrodes and lower electrodes, the thermoelectric element and the upper electrode or the lower electrode are joined to each other by using solder, and an electroless plating film of nickel having a resistivity of 10-60 μΩ·cm is formed on a joint surface of the thermoelectric element.
Japanese patent application publication JP-P2001-102645A (paragraphs 0006-0009) describes that a nickel plating layer having a thickness of 1-5 μm is apt to be formed with pinholes on a surface thereof, and as a result, a solder component diffuses into a thermoelectric semiconductor element through the pinholes. Accordingly, JP-P2001-102645A discloses a thermoelectric element formed with a nickel plating layer having a thickness of 7 μm or larger on a surface thereof in order to prevent diffusion of the solder component while keeping performance of the thermoelectric element.
Japanese patent application publication JP-A-H9-321352 (paragraph 0012 and FIG. 25) discloses a thermoelectric module having a thermoelectric element consisting of an element main body consisting of Bi—Te—Sb—Se, and a Ni layer and a Mo layer provided on a joint plane to be joined to a joining electrode. Further, JP-A-H9-321352 describes it is preferable that the Ni layer has a thickness of 1 μm or larger, and the Mo layer has a thickness of 1 μm or less.
Japanese patent application publication JP-P2008-10612A (paragraphs 0010-0012) discloses a method of manufacturing a thermoelectric element which method is capable of forming a diffusion prevention layer effective for preventing diffusion of elements and having a high peel strength on a thermoelectric material containing at least one of bismuth, tellurium, selenium, and antimony, and discloses a thermoelectric element manufactured by using such a method of manufacturing a thermoelectric element.
The thermoelectric element includes a thermoelectric material containing at least two of bismuth (Bi), tellurium (Te), selenium (Se), and antimony (Sb), a diffusion prevention layer formed on the thermoelectric material and for preventing diffusion of a different kind of element into the thermoelectric material, and a solder joint layer formed on the diffusion prevention layer and for joining the diffusion prevention layer and solder to each other, and is characterized in that a peel strength at an interface between the thermoelectric material layer and the diffusion prevention layer or an interface between the diffusion prevention layer and the solder joint layer is 0.6 MPa or more.
Japanese patent application publication JP-P2011-171668A (paragraphs 0013-0014) discloses a thermoelectric power module capable of bearing long time use in a high temperature environment where a temperature at a higher temperature part exceeds 250° C. The thermoelectric power module includes an thermoelectric power element, a first diffusion prevention layer disposed on a surface of the thermoelectric power element and consisting of molybdenum (Mo), a second diffusion prevention layer disposed on a surface of the first diffusion prevention layer opposite to the thermoelectric power element side and consisting of an intermetallic compound of nickel-tin (Ni—Sn), an electrode, a third diffusion prevention layer disposed on a surface of the electrode and consisting of an intermetallic compound of nickel-tin (Ni—Sn), and a solder layer joining the second diffusion prevention layer and the third diffusion prevention layer to each other and containing lead (Pb) at not less than 85%.